1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotography system or an electrostatic recording system. More particularly, the present invention relates to an image forming apparatus such as a copying machine, a printer or a facsimile machine.
2. Description of Related Art
Some recent copying machines are used as a multi function printer (MFP) while connected to a network together with a printer or other apparatuses. Under such an environment, color matching of printed images among the apparatuses connected to the network or matching of the color of images displayed on a display device such as a CRT and the color of printed images is frequently performed. For that purpose, various color management techniques are known.
For example, in the case of color management using an international color consortium (ICC) profile, calibration (i.e. color matching, which is sometimes referred to as characterization) is performed by creating apparatus's own ICC profile for a printer or a copying machine etc., and print data is generated by performing color conversion based thereon using, for example, a personal computer (PC). Then, the print data is output to an apparatus that is adapted to that profile so that matching of color of printed images and the color of images displayed on a display device or the like is performed.
Since software for creating a profile and color measuring device are commercially available to general users, environments that allow general users to perform matching of the color output by an image forming apparatus such as a printer with desired color is gradually realized. In another calibration process that is presently used, color conversion of an ICC profile using multi-dimensional LUT is not performed, but desired gradation characteristics are obtained by modifying the content of a gamma LUT concerning gradation.
As per the above, color management is an effective technique in reducing difference in the output color among a plurality of apparatuses of the same type or different types, and its scope of application is not limited to the above-described situation. For example, it is being applied to the case in which a printer is used for the purpose of color calibration of printing performed by an offset printing machine by matching the color printed by the offset printing machine with the color printed by the printer. For example, color management as shown in FIG. 22 is made possible with an application software on a PC by preparing the ICC profile of the printing machine and the printer respectively.
As shown in FIG. 22, the content of an ICC profile for the printing machine 51 and the content of an ICC profile for the printer 52 are calibrated in relation to a color space that is not depends on the printing machine or the printer, such as, for example, the CIE L*a*b* color space, based on color measurement of a patch(es) using a calorimeter. Thus, the color printed by the printing machine and the color printed by the printer can be matched with each other. In addition, the color management module (CMM) 53 can create print data by performing color conversion using those profiles.
As per the above, with development of color management environment such as calorimeters, applications and softwares for creating a profile, use of an image forming apparatus using an electrophotography system for the purpose of color calibration of a printing machine is prevailing mainly in the field of designing.
On the other hand, color adjustment of the copying machine's engine is performed by means of the process in which a monochrome gradation patch is formed, density is calculated by a reader part, and a one-dimensional LUT (gradation correction table) with which a desired target (such as linearity in density or linearity in brightness etc.) can be realized, as disclosed in Japanese Patent Application Laid-Open No. 11-75067.
Furthermore, as described in Japanese Patent Application Laid-Open No. 2002-72577 or No. 2002-72574, stability of color can be realized, without requiring user's efforts, by forming a patch image on a transferring member, detecting and converting attached toner amount (or toner density) by means of a regular reflection sensor, and feeding it back to a LUT (a gradation correction table for correcting the gradation of images) or an ATR (toner replenishment amount in an apparatus for automatically controlling toner replenishment amount to a developing device).
By the way, in comparing the image quality of a general color copying machine and an offset printing machine, it turns out that the difference in resolution is large. The resolution of CTP for producing a plate of offset printing is at least 2400 dpi. On the other hand, the resolution of copying machines (or printers) using electrophotography system is 600 dpi on the average. Consequently, there is the problem that small point letters, proportion of letters and family as variations of fonts cannot be formed.
In addition, in the case that a dithering matrix is created in 600 dpi resolution in a copying machine, it is difficult to form a screen equivalent to that of a printing machine, since the number of control points is small.
As per the above, conventional color copying machines suffer from many problems concerning the resolution, and there is a demand by users for an increase in the resolution. On the other hand, the color copying machine is not always used for color comprehensive layout or as a proofer, but also for office use. Thus, an increase in the resolution involves problems that are common to all users, such as an increase in the cost of the apparatus or an increase in the processing time, and it is not appropriate to increase the resolution regardless of the situation.
In view of the above problems, there has been proposed an apparatus in which resolution can be switched while keeping the increase in the cost as low as possible to meet the needs of both types of users.
In the technology disclosed in Japanese Patent Application Laid-Open No. 59-221165, the dot density of electrostatic latent images formed on a photosensitive drum can be varied, and for that purpose, the frequency of a laser beam (namely, the frequency of a laser power modulation signal) or rotation speed of a polygon mirror are adapted to be changed.
Furthermore, in the technology disclosed in Japanese Patent Application Laid-Open No. 60-7263, upon laser beam pulse width modulation, the rising time of a pulse is controlled in accordance with the pulse width to adjust dot intervals so as to realize an increase in the resolution.
Still further, in the technology disclosed in Japanese Patent Application Laid-Open No. 60-182868, the resolution is switched by means of a so-called twin beam laser system.
In addition, in the technology disclosed in Japanese Patent Application Laid-Open NO. 2000-181275, the process speed in 1200 dpi resolution is designed to be half of the process speed in 600 dpi resolution so that switching between the user that requires high printing speeds but does not require high resolutions and the user that requires fine images can be realized.
However, there is the problem that an increase in the resolution invites instability in printed images.
For example, in view of the present optical technologies and costs, the limit of the laser spot diameter is approximately 35 μm. On the other hand, the pixel size in 600 dpi resolution and in 1200 dpi resolution is 42.3 μm and 21.1 μm respectively. In the former case, the laser spot diameter is smaller than the pixel size, but in the latter case, the laser spot diameter is larger than the pixel size. Therefore, sharp, deep and stable latent images cannot be formed in the latter case, and an instability factor arises due to overlapping of the outskirt portions of latent images. Furthermore, in other processes, a highly fine development technology or a highly efficient transferring technology etc. is required and there is a difference in stability between a low resolution printing and a high resolution printing.
Furthermore, stability is greatly related not only to the basic resolution but also to the screen line number serving as an image forming pattern in the case of area coverage modulation. Specifically, due to the problem of stability of latent images, it is difficult to ensure stability as the line number increases.